5

Conversions and Promotions

CONVERSIONS AND PROMOTIONS

A conversion from type

double

 to type

long

 requires a nontrivial translation

from a 64 bit floating point value to the 64 bit integer representation.

Depending on the actual run time value, information may be lost.

In every conversion context, only certain specific conversions are permitted.

The specific conversions that are possible in Java are grouped for convenience of

description into several broad categories:

Identity conversions

Widening primitive conversions

Narrowing primitive conversions

Widening reference conversions

Narrowing reference conversions

String conversions

There are five

conversion contexts

 in which conversion of Java expressions

may occur. Each context allows conversions in some of the categories named

above but not others. The term  conversion  is also used to describe the process of

choosing a specific conversion for such a context. For example, we say that an

expression that is an actual argument in a method invocation is subject to  method

invocation conversion,  meaning that a specific conversion will be implicitly cho 

sen for that expression according to the rules for the method invocation argument

context.

One conversion context is the operand of a numeric operator such as

+

 or

*

.

The conversion process for such operands is called

numeric promotion

. Promotion

is special in that, in the case of binary operators, the conversion chosen for one

operand may depend in part on the type of the other operand expression.

This chapter first describes the six categories of conversions ( 5.1), including

the special conversions to

String

 allowed for the string concatenation operator

+

.

Then the five conversion contexts are described:

Assignment conversion ( 5.2,  15.25) converts the type of an expression to

the type of a specified variable. The conversions permitted for assignment are

limited in such a way that assignment conversion never causes an exception.

Method invocation conversion ( 5.3,  15.8,  15.11) is applied to each argu 

ment in a method or constructor invocation and, except in one case, performs

the same conversions that assignment conversion does. Method invocation

conversion never causes an exception.

52